This invention relates to television pick-up tubes and particularly to a mesh assembly having reduced microphonics for such a tube.
Microphonics is a physical vibration of some portion of a tube which, because of its movement produces an undesirable electrical signal output or noise which manifests itself as a background of lines or striations in a television picture.
In a vidicon type pick-up tube a mesh grid is disposed between a photoconductive target and an electron gun which provides a scanning electron beam. The mesh provides a lens action which causes the electron beam from the electron gun to impinge perpendicularly on the target electrode. The mesh is usually supported around its periphery by at least one annular support ring.
When such tubes are subjected to mechanical shock, for example in vehicles or equipment, and vibrations from, for example, cooling fans or incident sound from external sources, the conductive mesh will start vibrating relative to the photoconductive target and cause microphonic effects which produced the above-described background lines and striations in the picture.
Many expedients have been adopted to eliminate the undesirable microphonic effects. Among the expedients adopted is a structure described in U.S. Pat. No. 3,906,278 to Horton et al., issued Sept. 16, 1975 comprising a fine conductive mesh and an annular corrugated washer of flexible mesh material clamped between two annular members. The annular members are dished away from each other beyond the region of clamping. The flexible mesh washer extends inwardly beyond the region of clamping and contacts both the fine conductive mesh and the dished portion of one of the clamping members in order to damp vibrations. The Horton et al. structure is complex and requires a precisely formed flexible mesh washer. If the corrugations of the mesh washer are too shallow the washer will not contact both the dished annular member and the fine mesh. If this occurs, little or no damping will occur. Being constructed from mesh, the flexible washer also tends to undergo a change in elasticity after repeated thermal and mechanical cycling thus decreasing the effectiveness of the damping action. It is therefore desirable to find a low cost, reliable mesh mounting system which provides sufficient mesh taughtness and damping to reduce microphonics.